1. Field of Invention
The present invention relates to a piezoelectric oscillator used in various electronic machines. In particular, the invention relates to a piezoelectric oscillator which is loaded with a quartz crystal or other piezoelectric resonator element, a semiconductor IC element with an oscillation circuit to drive the resonator element, such as an IC chip, and the like and/or discrete electronic components.
2. Description of Related Art
Piezoelectric devices, such as a piezoelectric resonator utilized as a clock source of electronic circuits, oscillators in which a piezoelectric resonator and an IC chip are sealed in same package, and the like have been widely used in various electronic machines, including but not limited to OA machines, such as information communication machines, and computers, and the like, livelihood machines, and the like. Further, the piezoelectric devices must be miniaturized and thinned in order to miniaturize and thin these electronic machines, and many surface-mounting type piezoelectric devices suited for mounting to the circuit boards of the devices have been adopted. More recently, piezoelectric devices that are actuated at the high frequency of 80 MHz or over of the fundamental wave have been required to correspond to the high communication frequency and high speed of systems associated with the large capacity and high speed of information transmission in the field of information communication based on portable telephones.
For example, a related art surface-mounting type quartz crystal oscillator includes a hollow portion formed on a laminated circuit board formed of at least three insulating layers. IC chips and electronic components (resistors) are arranged in a cavity formed at the bottom of the hollow portion. A quartz resonator element is arranged in the upper part of the hollow portion. A cover is joined to the upper end of the laminated circuit board to seal it to be air-tight. Terminal electrodes for an external connection are formed on the rear face of the laminated circuit board. This structure is disclosed in Japanese Laid Open Utility Model H6-48216. In the surface-mounting type piezoelectric oscillator described in Japanese Laid Open Patent Application H11-186850, a reactance component due to wiring is reduced, and an enhancement of high-frequency characteristics is sought simultaneously with the miniaturization and thinning by receiving a quartz crystal resonator and its driving IC chip in the same case, directly connecting the driving IC chip with solder bumps to wiring patterns formed on the inner bottom of the case and electrically fixing/keeping the quartz crystal resonator element above it to electrically connect the element to the wiring patterns. In the quartz crystal oscillator described in Published Unexamined Patent Application H9-107186, an oscillating element and a control element are mounted on a ceramic board, and a resin case where external connection terminals are connected to electrodes of the ceramic board is externally arranged covers the ceramic board, and then is sealed to prevent wire breakage caused by differences in the thermal expansion coefficients during mounting to the mounting board of another machine.
In order to maintain high-precision frequency during the use of a piezoelectric oscillator, it is necessary to synchronize with other reference frequencies, and to always maintain the frequency at a desirable value or range. Moreover, deviations of the frequency that are caused by temperature changes and the like must be corrected to correspond to ambient changes, such as temperature, and the like. To realize this, for example, a function that is capable of fully correcting the frequency difference from the reference frequency with a variable capacitor changed with a control voltage and the like is required for the piezoelectric oscillator.
A piezoelectric oscillator includes a bottom wall defining a circuit pattern that is formed to connect a piezoelectric resonator element and an IC chip, and a circular sidewall with an opening to receive them are laminated and burned to form a ceramic base. An adjustment auxiliary portion to integrally extend the bottom wall and the circular sidewall is arranged on one end side of the ceramic base. Adjustment terminals to connect the circuit pattern are formed on an external surface of the ceramic. This structure is disclosed in Japanese Laid Open Patent Application H10-22735 as an example of TCXO (temperature compensated crystal oscillator). This piezoelectric oscillator reduces or prevents the occurrence of strain or warp in the manufacture of the ceramic base by writing compensation data in a memory circuit of the IC chip via the adjustment terminals, and then dividing the adjustment auxiliary portion.
An equivalent circuit model of such a voltage controlled crystal oscillator is shown in FIG. 5. The frequency change of a quartz crystal oscillator to the change of load capacity CL is expressed by the following equation (1). In the application of communication machines used at a very high frequency, such as portable telephones, the frequency variable width, i.e., xcex94F of the quartz crystal oscillator is as large as possible, therefore xcex3=C0/C1 is preferably decreased.                                           Δ            ⁢                          xe2x80x83                        ⁢            F                    =                      1                          2              ⁢                              xe2x80x83                            ⁢                              γ                ⁡                                  (                                      1                    +                                          CL                      C0                                                        )                                                                    ⁢                  
                ⁢                  Here          ,                      xe2x80x83                    ⁢                      γ            =                          C0              C1                                                          (        1        )            
However, an actual quartz crystal oscillator has a stray capacity Cpp as shown in FIG. 5. In this case, the apparent parallel capacity becomes (C0+Cpp). Therefore, the frequency change xcex94F of the quartz crystal oscillator is corrected as the following equation (2).                                           Δ            ⁢                          xe2x80x83                        ⁢                          F              xe2x80x2                                =                      1                          2              ⁢                              xe2x80x83                            ⁢                              γ                ⁡                                  (                                      1                    +                                          CL                                              C0                        +                                                  C                          pp                                                                                                      )                                                                    ⁢                  
                ⁢                  Here          ,                      xe2x80x83                    ⁢                                    γ              xe2x80x2                        =                                          C0                +                                  C                  pp                                            C1                                                          (        2        )            
From equation (2), it is determined that if the stray capacity Cpp increases, the frequency change xcex94F to the load capacity CL decreases, and that the higher the frequency, the greater the stray capacity Cpp. This represents a big problem because high accuracy is required for an AT cut quartz crystal resonator that is actuating at a very high frequency.
The capacity of the quartz crystal element itself, an capacity between lead-out electrodes that are formed on a quartz crystal resonator element, an capacity generated between terminals of a package and an capacity generated between the quartz crystal resonator element and the package, and the like are considered as the reasons for such a stray capacity. The inventors confirmed that the stray capacity correlating to the package is Cpp≈(nearly equal) C0 at 80 MHz or higher in various piezoelectric materials, especially in the case of quartz crystal. Therefore, its influence on the frequency change is large.
As with the quartz crystal oscillators described in Japanese Laid Open Patent Application H11-186850 and Japanese Laid Open Patent Application H9-107186, in a structure where a quartz crystal resonator and electronic components, such as an IC utilized to drive and the like are loaded in the same package, the stray capacity that is generated between them decreases and therefore is favorable. However, a concern exists that a gas generated from the bumps of electronic components and electrode pads, as well as an adhesive used to fix them, will be adsorbed on the surface of the quartz crystal resonator element, and will exert a negative influence on the vibration characteristics thereof. The decrement of frequency caused by the adsorption of such a gas is expressed by the following equation (3), and the higher the frequency, the greater the decrement. For example, where the same amount of gas is adsorbed on the quartz crystal resonator element, the frequency decrement 1 ppm at a frequency of 20 MHz becomes 25 ppm at a frequency of 100 MHz.                               Δ          ⁢                      xe2x80x83                    ⁢          f                =                                            W              ab                                                      p                q                            ·                              V                q                                              ·                      f            2                                              (        3        )            
where,
Wab: the weight of adsorbed gas on electrode surface (g/cm2)
Pq: the density of quartz crystal (g/cm3)
Vq: the propagation velocity in quartz crystal
f: the resonance frequency
Thus, the quartz crystal resonator that is actuating at a very high frequency is very sensitive to ambient gas and is easily affected. Therefore, it is desirable that other electronic components not be loaded in same atmosphere. For the quartz crystal oscillator described in Japanese Laid Open Patent Application H8-316732, in order to prevent the deviation of the resonance frequency caused by the adsorption of a gas that is generated from an adhesive to join a cover after sealing the case, a first receiver is arranged in the case and a second receiver is arranged at its bottom, an oscillation circuit, such as IC and the like, is received in the first receiver, a quartz crystal resonator element is loaded in the second receiver, and sealed with a cap and built into the case. Then the frequency is measured and the IC is adjusted in a state equivalent to the piezoelectric resonator. Then, the frequency is adjusted for residual strain in the manufacture of the quartz crystal resonator element and the strain caused by fixing it to the case. However, the quartz crystal oscillator disclosed in JP 732 is subject to the problem that the thinning and miniaturization of the entire package are restricted because another air-tight sealed empty chamber for the quartz crystal resonator is arranged in an air-tight sealed container to make the cover into a dual structure.
Accordingly, the present invention addresses the mentioned problems. The invention provides a piezoelectric device, in which a piezoelectric resonator element and electronic components to constitute its oscillation circuit are loaded in same package, and that effectively reduces or prevents the reduction of a variable amount of frequency caused by a stray capacity due to the adsorption of a gas that is generated from the electronic components and wiring, package or wiring structure, and the like, adjusts the frequency with high accuracy corresponding to the requirements of miniaturization and thinning, and is particularly suitable for uses of a high-frequency oscillator with a fundamental wave of 80 MHz or higher.
To achieve the above, the present invention provides a piezoelectric device that includes a package having a board to form wiring patterns on one principal plane with a first region and a second region, a first sidewall arranged on the board so as to enclose the first region and delimit a first internal void space and a cover to air tightly join to the upper end of the first sidewall and seal the first void space to be air-tight, a piezoelectric resonator element connected to the wiring patterns and mounted in the first void space and electronic components connected to the wiring patterns and mounted in the second region. The electronic components and the wiring patterns of the second region are resin sealed.
Thus, high frequency stability and aging characteristics are obtained by sealing the piezoelectric resonator element to be air-tight in the first void space that provides an atmosphere that is different from the electronic components because of no stray capacity caused by the adsorption of a gas that is generated from the electronic components and the like after sealing. There is also no occurrence of stray capacity, as in a wiring structure in which the wiring patterns formed on multiple laminated sheets are interlayer connected, on account of a wiring structure in which the piezoelectric resonator element and the electronic components are directly connected to the wiring patterns that are formed on the board which is the bottom plate of the package. Thus, there is no concern of producing a deviation of frequency caused by such a stray capacity. Moreover, frequency adjustment can be made with high accuracy by adjusting the electronic components of the second region in a state in which the piezoelectric resonator element is sealed to be air-tight. Furthermore, unlike the quartz crystal oscillator that is disclosed in Japanese Laid Open Patent Application H8-316732, thinning and miniaturization of entire package is possible because there is no need to make the cover into a dual structure.
The above structure provides the following advantages.
In accordance with the present invention, since the piezoelectric resonator element is air-tightly sealed in the first void space with an atmosphere that is isolated from the electronic components, no adsorption of gas that is generated from the electronic components exists. Moreover, since the piezoelectric resonator element and the electronic components are directly connected to the wiring patterns that are formed on the board, there is no concern that the stray capacity occurs and causes deviation in frequency due to the gas adsorption caused by such a piezoelectric resonator element and the wiring structure of package. Accordingly, the present invention enables the display of high frequency stability and aging characteristics as well as high-accuracy frequency adjustment by adjusting the electronic components of the second region. A surface-mounting piezoelectric device that can be miniaturized and thinned is thus obtained. Particularly, the present invention enables the increase of the variable width of frequency in the case of applying it to a voltage controlled high-frequency oscillator, and thus it is favorable for use in communication apparatus.